The invention relates to a thermal radiation detector comprising means, having a temperature-dependent characteristic, for detecting thermal energy. The detecting means are supported by at least one flexible film which is substantially transparent to the radiation to be detected. With respect to the direction of incidence of the thermal radiation to be detected, the flexible film precedes the detecting means. The invention further relates to an array of such detectors, and to a method of operating a thermal radiation detector.
A detector as described above, wherein the detecting means comprises an element of pyroelectric material, is described in European Patent Application EP No. 41,297 (corresponding to U.S. Pat. No. 4,425,502). In one form of infrared detector described therein, the element of pyroelectric material is supported by and between two flexible plastic films which transmit approximately 80% of incident radiation in the wavelength range of 8-14 .mu.m. The detector receives infrared radiation via one of the plastic films. The plastic films support electrically conductive layers providing electrical connections to the pyroelectric element. This arrangement enables the detector to have particularly low susceptibility to microphony as well as low lateral thermal conductance.
A problem with thermal radiation detectors is absorbing sufficient incident radiation. For example, pyroelectric materials suitable for good-quality detectors typically have rather low absorption over at least part of the wavelength range for which it is desirable to provide such detectors. Furthermore, the thinner the detector (a feature which is desirable to reduce thermal capacitance as well as thermal conductance to the surroundings), the lower will be the overall absorption of the incident radiation by the pyroelectric material.
A conventional way of improving absorption has been to blackened the detector. However, appropriate materials have the disadvantages of being awkward to apply and have poor adhesion.
The paper entitled "The application of thin film absorber coatings to enhance the sensitivity of fast pyroelectric detectors" by Peter C. LaDelfe et al (SPIE, Vol. 380 (1983), pages 266-273) refers to the application to a pyroelectric detector of a three-layer coating consisting of a dielectric layer sandwiched between two metal layers. Such coatings are described by Hilsum (in "Infrared Absorption of Thin Metal Films," Journal of the Optical Society of America, Volume 44, No. 3 (1954), pages 188-191) and Silberg (in "Infrared Absorption of Three-Layer Films," Journal of the Optical Society of America, Volume 47, No. 7 (1957), pages 575-578). See, also "Reflection and Transmission Interference Filters" by L. N. Hadley and D. M. Dennison (Journal of the Optical Society of America, Volume 37, No. 6 (1947), pages 451-465.
The paper by LaDelfe et al refers to the work of Annis and Simpson ("Absorption of Radiation in PLZT Pyroelectric Detectors", Infrared Physics, Volume 14 (1974), pages 199-205). Annis and Simpson applied the theoretical work of Hadley & Dennison, Hilsum and Silberg to a three-layer structure wherein the dielectric sandwiched between the two metal layers was the pyroelectric material itself and had a thickness of 10-15 .mu.m , giving an optical thickness of roughly 21/2 wavelengths in a typical operating range. LaDelfe et al themselves applied a two-layer coating consisting of a metal layer and an antireflection layer to chips of pyroelectric material 0.25 mm thick.